How to blur a texture node in cycles material?
I have found that b°wide NodePack contains a blur node! It's called ImageBlur.

What it does is play with the vectors to blur it, something like if a ray comes it collide to multiple points (effectively blurring the texture). I don't know exactly how, but here is a screen capture of the node expanded.



Note that in the background I have used it for testing in a ImageTexture node with a camera vector, that is processed by the ImageBlur and then passed to the ImageTexture vector input.

You can see all its nodes for your use (it is in public domain) from b°wide nodepack for blender and you can download it in blendswap. It have many usefull nodes.